1. Field of the Invention
The present invention relates to phase noise control, and more particularly, to a Voltage Controlled Oscillator (VCO) for controlling phase noise and a method of controlling the phase noise by using the VCO.
2. Description of the Related Art
A Voltage Controlled Oscillator (VCO) is an apparatus for outputting a desired oscillation frequency using a supply voltage, and is widely used, for example, in mobile communication terminals and the like. For example, the VCO functions as a local oscillator, which may be used in a Phase Locked Loop (PLL) module of a mobile communication device to assign channels and to convert a frequency into a Radio Frequency (RF) or an Intermediate Frequency (IF).
Complementary metal-oxide-semiconductor (CMOS) VCOs, including LC resonators, are generally used as high-frequency VCOs. As many types of mobile communication applications have been recently developed, and as low-power system ICs having many integrated functions have become more prevalent, CMOS circuits have become widely used due to their low manufacturing costs, integratibility, and high reliability in the manufacturing process. However, since the CMOS circuits generally have poor noise characteristics, a low breakdown voltage, and a low gain, CMOS circuits present some difficulties notwithstanding their high integration and efficient productivity.
Accordingly, improving phase noise performance has been actively researched in order to maximize the VCO performance, while maintaining the advantages of CMOS circuits. An exemplary VCO is disclosed in U.S. Patent Application Publication No. 2004/0085144, which published on May 6, 2004.
FIG. 1 is a circuit diagram of a conventional VCO 100 using an LC resonator 150.
Referring to FIG. 1, the conventional VCO 100 includes differential amplifiers 110 and 170, and an LC resonator 150. The differential amplifier 110 is a latch circuit in which two PMOS transistors P110 and P115 are cross-coupled. That is, the gate of the PMOS transistor P110 is connected to a first terminal of the PMOS transistor P115, a first terminal of the PMOS transistor P110 is connected to the gate of the PMOS transistor P115, and a second terminal of the PMOS transistor P110 is connected to a supply voltage source VDD. Likewise, the gate of the PMOS transistor P115 is connected to the first terminal of the PMOS transistor P110, the first terminal of the PMOS transistor P115 is connected to the gate of the PMOS transistor P110, and the second terminal of the PMOS transistor P110 is connected to the supply voltage source VDD.
The differential amplifier 170 is also a latch circuit in which two NMOS transistors N170 and N175 are cross-coupled. The differential amplifier 170 has substantially the same construction as the differential amplifier 110, except using NMOS transistors N170 and N175 instead of PMOS transistors.
The LC resonator 150 is connected between the differential amplifiers 110 and 170. The LC resonator 150 determines a frequency by varying an inductance component or a capacitance component. The points at which the differential amplifiers 110 and 170 are connected to the LC resonator 150 are used as output terminals Vo+ and Vo− of the VCO 100.
As stated above, as many types of mobile communication applications have been developed, requiring low-power system ICs with many integrated functions, a VCO capable of improving phase noise performance is needed.